In prior art devices of the type to which the invention is directed, an exposure factor setting member is locked to prevent movement thereof from a specific set position. However, such prior art devices are arranged such that the locking mechanism automatically operates to always lock the exposure factor setting member in a specific position, regardless of the photographer's intention, when the exposure factor setting member is merely shifted to the specific position. For example, a camera capable of shutter speed priority automatic exposure control requires a diaphragm aperture setting member to be set at a specific position, such as the minimum diaphragm aperture position, when photographing in such an automatic exposure control mode. Thus, such known cameras have the disadvantage that correct exposure is unobtainable if the diaphragm aperture setting member should move from the specific set position, and it would be preferable therefore to lock the diaphragm aperture setting member in the specific set position.
In prior art diaphragm aperture setting mechanisms, the lock mechanism automatically operates to lock the diaphragm aperture setting member in the set specific position. In such diaphragm aperture setting mechanisms, however, to enable photography in any control mode other than automatic exposure control, e.g., photography with the diaphragm aperture setting member manually set to any desired position, the lock mechanism inevitably operates to lock the diaphragm aperture setting member when it reaches the specific position as it is being rotated for selecting the diaphragm aperture, thereby impeding the diaphragm aperture selecting operation. Consequently, prior art lockable diaphragm aperture setting mechanisms are disadvantageous in that a photographer has to operate the lock mechanism for releasing such abrupt locking, resulting in missing photographic opportunities.